The control of the intensity of light is one factor considered in the design of displays and lighting. Errors in the control of light intensity may result in visual defects noticeable to a viewer (e.g., an off color pixel that occurs in an image area of even color and brightness). A number of methods of controlling the light intensity that are subject to such errors are described below. These methods fall generally into two types: pulse width modulation (PWM) and variable pulse frequency.
PWM, also referred to as a pulsed duty cycle, generally requires that the width or duration of a pulse is varied in length to control the current supplied to a light source. Typically, the longer the pulse duration, the longer the current flows through the light source. According to this method, the associated electronic circuitry changes the rise and/or the fall times of the pulse to accomplish the variation in pulse length. One disadvantage of PWM is that the total flow of current is not entirely a function of pulse length. Capacitance and inductance of the circuit controlling the light source affect the flow of current for the duration of the pulse length. In addition, this effect is not a constant value but varies at each discrete moment of time during the pulse. As a result, a pulse of twice the duration in length of a first pulse does not have twice the total current flow of the first pulse.
In another method, the frequency of the pulse within a time period may be varied to control the current supplied to a light source. Generally, increasing the frequency of pulses within the time period produces more total current resulting in greater brightness or intensity of the light source. Reducing the frequency of pulses within the time period produces less total current resulting in reduced brightness or intensity of the light source. Frequency generation is commonly achieved using a voltage controlled oscillator (VCO). In one example, a voltage reference across a capacitor may be varied to control the frequency output by an oscillator. The resultant frequency provided from the VCO is used to produce pulses that allow current to flow through the light source. A drawback of this method is that the analog circuitry used to create the voltage reference reduces the overall accuracy and preciseness of timing. However, even when frequency variation is generated using a digital source, a precise frequency may not be achieved because frequency generation is a reciprocal of time, and the reciprocal of any prime number is not evenly divisible over a period of time.